1. Field of the Invention
The present invention relates to modules that compensate for chromatic dispersion of light signals when transmitted through an optical fiber.
2. Discussion of the Known Art
Dispersion compensating modules (DCMs) including one or more lengths of dispersion compensating fibers (DCFs), are generally known as a means for compensating for chromatic dispersion of light signals when transmitted through a fiber of a fiber optic communication system. DCMs thus enable existing systems to handle signals with bandwidths and wavelengths for which the systems were not originally designed, and without the need for replacing long spans of installed fiber optic cable with newer, higher rated cables. As transmission bandwidth requirements increase, the range of wavelengths over which a DCM must provide effective dispersion compensation will therefore increase accordingly.
The use of more than one type of fiber in a DCM has certain advantages, including tighter manufacturing tolerances and simultaneous control of dispersion, dispersion slope, and even higher order dispersion over a broad bandwidth (50 nm or more). Because DCMs may themselves use several kilometers of fiber having a certain signal attenuation factor, it is useful to combine the functions of dispersion compensation and Raman amplification into a single, Raman amplified, dispersion compensating module or xe2x80x9cRADCMxe2x80x9d.
As signal transmission wavelengths approach regions that cannot be handled by known rare-earth doped amplifiers, or as the transmission bandwidth exceeds that of current erbium-doped optical amplifiers, the need for a discrete amplifier with broadband gain becomes critical. Discrete Raman amplifiers have an advantage in that (a) they may operate in any wavelength range depending only on a supplied pump wavelength, and (b) they can achieve a broad gain-bandwidth product by using multiple pumps at several wavelengths.
Existing DCMs having excellent dispersion properties are not always capable of being modified into efficient RADCMs, however. For example, a given DCM may provide good dispersion compensation but not be able to provide enough gain for the available Raman pump power. Also, fibers used in the DCM for dispersion compensation may be too long and cause noise due to multi-pass interference (MPI), or the fibers used in the DCM for dispersion compensation may have too small an effective area and cause undesired four-wave mixing (FWM).
As mentioned, the concept of Raman pumping a single fiber DCM to compensate for signal loss is generally known. See, e.g., U.S. Pat. No. 5,887,093 (Mar. 23, 1999) all relevant portions of which are incorporated by reference. In an article by S. A. E. Lewis, et al., in 36 Elec. Lett. (2000) at page 1355, a broadband RADCM is described with two fibers whose lengths are carefully chosen together with the amount of pump power supplied to each fiber. To maintain a low noise figure, the fiber which provides the bulk of dispersion compensation is given relatively little pump power and contributes less than 25% of the gain. Notwithstanding, the predominantly compensating fiber contributes more than half of the noise. In addition, a complicated mid-span pump arrangement with multiple circulators is required. Thus, while Lewis, et al. demonstrate that Raman gain and dispersion compensation may be achieved independently, the article does not show that desired dispersion compensation and Raman gain can be achieved simultaneously, or that the DCF may achieve either wideband or slope and curvature compensation.
A multi-stage amplifier with the option of using DCFs is described in U.S. Pat No. 6,335,820 (Jan. 1, 2002), all relevant portions of which are incorporated by reference. Low noise operation was achieved by the use of a mid span lossy element such as an optical isolator, and a mid-span pumping configuration, however.
In view of the known art, there remains a need for a RADCM that provides sufficient gain, dispersion, and dispersion slope compensation with low noise due to DRS, MPI and FWM, and without the need for any complex mid-span pumping schemes or loss elements.
According to the invention, a dispersion compensation module of the kind having at least two dispersion compensating fibers to compensate for chromatic dispersion produced in light signals conducted through a transmission fiber, includes a first dispersion compensating fiber (DCF) having a first length, an input end and an output end, and the first DCF has a first Raman gain coefficient (gR(xcex)), a first Raman effective fiber area (Areff), and a first dispersion characteristic. The module also has a second DCF having a second length, an input end and an output end, wherein the input end of the second DCF is arranged to receive light signals from the output end of the first DCF and in the absence of a pump signal source between the input end of the second DCF and the output end of the first DCF. The second DCF has a second Raman gain coefficient, a second Raman effective fiber area, and a second dispersion characteristic selected to cooperate with the first dispersion characteristic to produce a desired module dispersion that compensates for chromatic dispersion produced in the light signals when conducted through the transmission fiber and applied to the input end of the first DCF.
A pump light source is coupled to either the output end of the second DCF or to the input end of the first DCF. The pump light source has a certain power level at one or more wavelengths to produce a desired module gain with a determined bandwidth for amplifying the light signals, and the lengths of the first and the second DCFs are selected in a manner that optimizes the module gain while maintaining the desired total module dispersion.
For a better understanding of the invention, reference is made to the following description taken in conjunction with the accompanying drawing and the appended claims.